1. Field of the Invention
The present invention relates to a battery charging apparatus and full-charging detecting method, and more particularly to a battery charging apparatus including a full-charging detecting means for detecting the full-charged state of a battery and a method of detecting the full-charged state of the battery.
2. Description of the Related Art
Generally, the battery such as a battery loaded in a vehicle (hereinafter referred to as "in-vehicle battery" will suffer degradation if charging therefor is continued in its full-charged state (remaining level of 100% of the battery). Therefore, in order to prevent the degradation of the battery due to excess charging, it is necessary to detect the full-charged state so that warning informing the full-charged state can be issued, or charging can be automatically stopped.
In order to realize this, a charging/discharging circuit has been proposed in which a battery charging device is built as shown in FIG. 7.
In FIG. 7, a battery charging device 1 includes an AC/DC converter 10 (AC/DC converting means) serving as a DC power source, and a full-charging detecting device 11 serving as a full-charging detecting means. The full-charging detecting device includes a current sensor 101, a voltage sensor 102 and a microcomputer 103.
While the in-vehicle battery is charged, the AC/DC converter 10 converts the AC voltage source from an alternator (not shown) into a DC voltage, and also supplies a charging current Ic to the battery B. The charging current Ic is also supplied to the current sensor 101 connected in series with the battery B.
On the other hand, while the in-vehicle battery B is discharged, a discharging current Io discharged from the in-vehicle battery B is supplied to a load 12 and also to the current sensor 101.
The current sensor 101 detects the charging current Ic supplied to the battery B and the discharging current Io produced therefrom, and supplies the detected values to the microcomputer 103. The microcomputer 103 is also supplied with the detected value of the terminal voltage across the battery B (hereinafter referred to as a terminal voltage VB) which is detected by the voltage sensor 102 connected in parallel to the in-vehicle battery B.
The microcomputer 103 includes a CPU 103a which is operated in accordance with a prescribed control program, an ROM 103b which stores the control program for the CPU 103a and a prescribed full-charging level, and a RAM 103c which temporarily stores the data necessary to perform the computation in the CPU 103a.
An explanation will be given of the operation of the charging/discharging circuit incorporating the battery charging device 1 having the configuration described above. While the in-vehicle battery B is charged, the CPU 103a successively captures the charging current Ic detected by the current sensor 101 and the terminal voltage VB detected by the voltage sensor 102 to compute an electric power (=Ic.times.VB) and detects an accumulated value of the electric power thus computed as a remaining level. On the other hand, while the battery B is discharged, the CPU 103a successively captures the discharging current Ic detected by the current sensor 101 and the terminal voltage VB detected by the voltage sensor 102 and computes an electric power (=Io.times.VB) on the basis of.them and subtracts an accumulated value of the electric power thus computed from the remaining level.
The CPU 103a successively detects the remaining capacity of the in vehicle battery B by the above accumulation system. When the detected remaining capacity exceeds the prescribed full-charged capacity held in the ROM 103b, the CPU 103a detects that the battery B has reached the full-charged state, and supplies a charging stopping signal Sl to the AC/DC converter 10. At this time, the AC/DC converter 10 stops supply of the charging current Ic to the battery B.
In short, when the in-vehicle battery B reaches the full-charged state, the AC/DC converter 10 stops to supply the charging current. Thus, the degradation of the battery B due to excess charging can be prevented.
Meanwhile, generally, the in-vehicle battery has a characteristic that it will gradually suffers degradation while the charging/discharging therefor is repeated. Particularly, a Ni-family battery that has been generally used as a battery for an electric vehicle has a great change in the full-charging capacity due to the degradation.
However, when the detected remaining capacity exceeds the prescribed full-charging capacity, the charging/discharging circuit incorporating the conventional battery charging device detects the full-charged state of the battery and simultaneously stops the charging. Therefore, if the actual full-charging capacity of the battery lowers owing to the degradation, the prescribed full-charging capacity becomes larger than the actual full-charging capacity so that the full-charged state of the battery cannot be detected accurately. As a result, when the battery suffers degradation, although the battery has already been fallen in the full-charged state, the remaining capacity in the full-charged sate does not exceed the prescribed full-charging capacity and the charging for the battery is continued. This leads to further degradation of the battery.
Meanwhile, the progression of degradation of the battery is proportional to the using period of the battery. Therefore, in order to obviate the above disadvantage, it can be proposed to form the map data of a plurality of full-charged capacities corresponding to the using period, which are to be used according to the using period. However, this proposal, which requires a memory having a large capacity enough to store a plurality of full-charging capacities, is problematic in terms of cost.
Further, the full-charging capacity of the battery changes with a change in the battery temperature. Specifically, as the battery temperature falls, the full-charging capacity decreases, whereas as the battery temperature rises, the full-charging capacity increases. Therefore, in the conventional battery charging apparatus, the actual full-charging capacity changes according to the battery temperature and hence becomes different from the prescribed full-charging capacity so that the full-charged state of the battery cannot be detected accurately.
Where the battery is successively used in an environment at a high temperature, the actual full-charging capacity increases with a rise in the battery temperature. As a result, although the battery has not been fully charged, the remaining capacity exceeds the prescribed full-charging capacity. Thus, the charging for the battery is stopped since the prescribed full-charging capacity&lt;actual full-charging capacity). On the other hand, where the battery is successively used in the environment at a low temperature, the actual full-charging capacity decreases with a fall in the battery temperature. As a result, although the battery has been already fully charged, the remaining capacity does not exceed the prescribed full-charging capacity. Thus, the charging for the battery is continued by the DC power source since the prescribed full-charging capacity&gt;actual full-charging capacity. This promotes the degradation of the battery.
As described above, the full-charging capacity changes with both the progress of degradation and the temperature of the battery. Therefore, in order to compensate for such changes, it is necessary to measure the using period of the battery and the temperature thereof and acquire the full-charging capacity on the basis of these measured values. This requires complicated control and computation.